20 POPULAR LECTURES AND ADDRESSES. 



work done in letting the two bodies come together 

 directly so as to meet in a unit area of each. In 

 the particular case of two similar bodies coming 

 together into perfect contact, the interfacial 

 tension must be zero, and therefore the work 

 done in letting them come together over a unit 

 area must be exactly equal to twice the surface- 

 tension ; which is the case we first considered. 



If the work done between two different liquids 

 in letting them come together over a small area, 

 exceeds the sum of the surface-tensions, the 

 interfacial tension is negative. The result is an 

 instantaneous puckering of the interface as the 

 commencement of diffusion, and the well-known 

 process of continued inter-diffusion follows. 



Consider next the mutual attraction between 

 a solid and a liquid. Choose any particular 

 area of the solid, and let a portion of the surface 

 of the liquid be preliminarily shaped to fit it. 

 Let now the liquid, kept for the moment rigid, 

 be allowed to come into contact over this area 

 with the solid. The amount by which the work 

 done per unit area of contact falls short of the 



